Does my scale need to be Trade Certified? 

All scales that are used for selling goods by weight need to be certified for trade use by an accredited person (A.P.). Not all scales are able to be certified for trade use, they must be of a type approved in New Zealand by the Ministry of Consumer affairs.

A test should be carried out by an A.P. every 12months or immediately if the scale has been repaired or recalibrated and then once the test has been completed a certificate of accuracy attached to the scale

Please refer to the government web site - http://www.consumeraffairs.govt.nz/for-business/compliance/accurate-measures/weights-and-measures-for-retailers